Oxidation smelting of base metal sulfide materials, including concentrates, has become a useful process which has been adapted in many countries for the treatment of a variety of sulfide material. Metal sulfide materials which may be treated by this technique contain a variety of valuable metals including copper, nickel, cobalt, lead, zinc, etc. Usually valuable metal sulfide ores and concentrates will also contain large amounts of iron sulfides such as pyrite and pyrrhotites and can contain undesirable impurities such as arsenic, bismuth, etc. Sulfide mineralization frequently occurs in admixture, e.g., copper with zinc and/or lead, copper with nickel, etc. Metal sulfide concentrates generally are finely divided.
In oxidation smelting the finely divided metal sulfide material, such as sulfide concentrate, in admixture with a flux material for iron oxide, e.g., silica, is first dried to eliminate water and then is injected along with an oxygen containing gas which can be oxygen enriched air or commercial oxygen by means of a suitable device which may be a burner. Part of the iron and sulfur contents of the concentrate burn with the combustion being supported by oxygen in the gas injected with the result that combustion preferably is autogenous. For example, in autogenous flash smelting the mixture of concentrate plus oxygen or oxygen enriched air is injected into a refractory furnace in a manner such that the oxidation of the sulfide occurs in the freeboard space of the furnace and the molten products of the combustion fall into the hearth of the furnace. The valuable metals are collected in the matte phase. The oxidized iron is fluxed by the silica to form a slag which collects on top of the molten matte. As desired, the matte and slag can be tapped at intervals. The process affords a means for smelting large quantities of sulfides on a continuous basis with generation of an off gas which can be 80% or more sulfur dioxide when the oxidizing gas consists of 100% commercial oxygen. The rich off-gas lends itself readily to treatment for recovery of liquid sulfur dioxide or for manufacture of sulfuric acid thereby making the operation highly advantageous for an environmental aspect. Another advantage of the process resides in the fact that the fuel for the process is iron sulfide which itself is not particularly valuable.
There is a well established prior art in regard to oxidation smelting and the technique is used throughout the world. As examples Canadian Pat. Nos. 503,446 and 934,968 may be mentioned together with the book "The Winning of Nickel" by J. R. Boldt and P. Queneau, Longman's, Canada, at pages 244 to 247 and various articles including the paper, "Oxygen Flash Smelting in a Converter" by M. C. Bell, J. A. Blanco, H. Davies and R. Sridhar, J. of Metals, Vol. 30, No. 10, pages 9-14, 1978; "Smelting Nickel Concentrates in Inco's Oxygen flash Furnace", by M. Solar et al., 107th AIME Annual Meeting, Denver, Colorado, Feb. 26-Mar. 2, 1978, "The KIVCET Cyclone Smelting Process for Impure Copper Concentrates" by Melcher, E. Muller and H. Weigel, J. of Metals, July 1976, pages 4-8; Paper by T. Nagano and T. Suzukii "Commercial Operation of Misu-Bishi Continuous Copper Smelting and Converting Process", published in Extractive Metallurgy of Copper, edited by J. C. Yannopoulas and J. C. Agarwal, the Metallurgical Society of AIME, 1976, Vol. 1, pages 439-457.
It is found that with any particular oxidation smelting furnace, it is necessary to arrive at a thermal equilibrium which is dependent upon the proportion of the sulfide concentrate burned. The heat generated by the combustion of the furnace feed, essentially of labile S and FeS to SO.sub.2 and iron oxides, equals the heat content of the smelting products (matte, slag and off-gas) plus the furnace heat losses. This means that, for a given sulfide material and a given furnace, a sufficient amount of oxygen per unit weight of sulfides must be supplied to satisfy the heat balance of the operation. When this is done, the matte grade is fixed, and the amount of oxygen cannot be altered without producing either an excess or deficiency of heat. In other words, the furnace balance, all other things being equal, determines the matte grade or the overall degree of conversion of the sulfide materials into a final product. This rigid interdependence of heat balance and degree of conversion is an important limitation of these processes. The present invention is directed to a means for controlling matte grade in oxidation smelting, e.g., autogenous flash smelting, at will.
The above mentioned interdependence of heat balance and degree of conversion of the concentrate in oxidation smelting, in particular autogenous oxygen flash smelting, makes it difficult to obtain the desired matte grades, especially when the concentrate has a low copper content and a high iron content. The interdependence of heat balance and matte grade applies to all of the aforementioned oxidation smelting processes.
It is to be appreciated that in the smelting of copper, as an example, the matte generated in the smelting furnace must be subjected to further treatment to provide blister copper which can in turn be transformed into high purity copper products. The smelting furnace matte grade controls the supplementary operations which must be performed downstream so as to arrive at blister copper. Thus, the higher the grade of the smelting furnace copper matte, the less needs to be done in converters or other equipment so as to provide blister copper and the less difficult are the problems in meeting environmental standards in regard to the evolution of sulfur dioxide in such downstream operations. In some cases, it may be desirable for example to provide a matte from the smelting furnace having the composition of white metal, almost pure Cu.sub.2 S.
A number of methods have been proposed for controlling the matte grade in oxygen flash smelting. Among these are: adding to the concentrate revert materials, such as dust, ground matte and slag skulls, etc.; water injection into the smelting unit; air dilution of the oxygen. All these alternatives consist of introducing a coolant into the smelting unit to use up the excess heat generated when a matte grade higher than that normally obtained in autogenous flash smelting is desired. They provide a way of achieving the same end result as the process of the present invention but they are not as attractive because higher oxygen additions are required and the processes become wasteful in energy utilization.